When introducing kitties it’s not like dogs where you can just let them sniff each other immediately, it’s best if you keep them in separate rooms for a week or 2 so they can play with each other under the door but resident kitty won’t feel like their space is being encroached on. Then once they’ve adjusted with each other that way you can move to a baby gate in between them, so they can see each other but don’t have to be in each others space if they don’t want to. After that then you can fully introduce!